as a method of "hugelkultur", by putting them in the soil either under or around whatever you are planting, hollow side up. This would create little "buckets" to hold water, and slowly rot away over time, normally by which time the tree will be more able to retrieve its water from deeper in the subsoil. Cheers.
Being a smart ass beats the alternative. This tiny ad knows what I'm talking about:
It's a bit like "run away to the circus" but for gardeners
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